Process for manufacturing a semiconductor power device comprising charge-balance column structures and respective device

ABSTRACT

Process for manufacturing a semiconductor power device, wherein a trench is formed in a semiconductor body having a first conductivity type; the trench is annealed for shaping purpose; and the trench is filled with semiconductor material via epitaxial growth so as to obtain a first column having a second conductivity type. The epitaxial growth is performed by supplying a gas containing silicon and a gas containing dopant ions of the second conductivity type in presence of a halogenide gas and occurs with uniform distribution of the dopant ions. The flow of the gas containing dopant ions is varied according to a linear ramp during the epitaxial growth; in particular, in the case of selective growth of the semiconductor material in the presence of a hard mask, the flow decreases; in the case of non-selective growth, in the absence of hard mask, the flow increases.

PRIORITY CLAIM

The present application claims priority from International Patent Application Serial No. PCT/IT2006/000244, published in English, filed Apr. 11, 2006, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

An embodiment of the present invention relates to a process for manufacturing a semiconductor power device and to the corresponding device. More specifically, an embodiment of the invention relates a process for manufacturing a power device of the type comprising column structures used for charge balancing of the device and to the corresponding device.

An embodiment of the invention regards, in particular, but not exclusively, a process for manufacturing a vertical-conduction power device (for example, a power MOS device or else a device of the Insulated-Gate Bipolar Transistor (IGBT) type or else a device of the Bipolar Junction Transistor (BJT) type or else bipolar diodes or Schottky diodes) and to the corresponding device, and the following non-limiting description relates to this application field for the purpose of explaining related concepts.

BACKGROUND

In the last few years the endeavor has been to increase the efficiency of the above devices for increasing their breakdown voltage and reducing their output resistance.

U.S. Pat. Nos. 6,586,798 B1, 6,228,719 B1, 6,300,171 B1 and 6,404,010, all commonly assigned and incorporated by reference, describe methods for solving the problems set forth above. These patents describe power MOS devices comprising column structures having a first conductivity type (for example, a P type), spaced by epitaxial layer portions having a second conductivity type (here, an N type). Basically, in order to form the column structures, a plurality of epitaxial layers of N type are sequentially grown, each growth being followed by an implantation of dopant of P type. Stacking of the implanted regions thus forms column structures, which represents an extension of the body regions of the device within the epitaxial layer that constitutes the drain region of the device. The charge concentration of the dopant introduced via the implantations, i.e., the concentration of the column structures of P type thus formed, is equal to, but of opposite sign with respect to, the concentration of charge introduced epitaxially. Consequently, by virtue of the charge balance thus obtained, vertical-conduction power devices can be provided with high breakdown voltage and with low output resistance, on account of the high concentration of the epitaxial layer (MD technology).

In addition, it is also known that, by increasing the density of the elementary strips that form the device, i.e., by increasingly packing the device, it is possible to further increase the charge concentration of the epitaxial layer, obtaining devices which, given a same breakdown voltage (linked to the height of the columns), have increasingly lower output resistance.

On the other hand, however, in order to increase the density of the elementary strips of the device, one increases the number of steps of epitaxial growth and reduces the thermal change undergone by the device. This may bring about an increase in the device cost, in the cycle time, and in the defects created by the various steps of epitaxial growth. In particular, with reference to the latter problem, each defect left at the interface between one epitaxial layer and the subsequent one may render the device less efficient.

Moreover, solutions exist for obtaining charge balance by forming trenches and filling the trenches with successive polysilicon, thermal oxide, and deposited oxide layers. However, these solutions may not solve the problem of defectiveness.

SUMMARY

An embodiment of the present invention is a process for manufacturing a power device of the type indicated above that may solve problems of known solutions in a simple way and may reduce to a minimum the defectiveness present in the device.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:

FIGS. 1 to 9 are cross-sections through a semiconductor wafer in successive manufacturing steps of the device, according to a first embodiment of the invention;

FIGS. 10 to 15 are cross-sections through a semiconductor wafer in successive manufacturing steps of the device, according to a second embodiment of the invention;

FIGS. 16 to 21 are cross-sections through a semiconductor wafer in successive manufacturing steps of an MD-trench device according to an embodiment of the invention;

FIG. 22 is a cross-section of a variant of the device of FIG. 21 according to an embodiment of the invention;

FIG. 23 is a cross-section of another variant of the device of FIG. 21 according to an embodiment of the invention.

DETAILED DESCRIPTION

[16] FIG. 1 shows a wafer 1 of semiconductor material, typically silicon, comprising a substrate 2 of an N++ type, with a resistivity lower than, for example, 10 mΩ, and an epitaxial layer 3 of N++ type, with a resistivity comprised approximately between 2Q and 10 mΩ. The wafer 1 has a surface orientation <100>. In addition, a hard-mask layer 5, for example of deposited silicon oxide, has already been provided on the top surface 4 of the epitaxial layer 3.

Then (FIG. 2), the hard-mask layer 5 is defined, by depositing a resist layer (not shown) and performing a lithographic etch and a dry etch so as to provide a hard mask, designated again by 5, formed by strips separated from each other by windows 6, which are also strip-shaped.

Using the hard mask 5, an anisotropic dry etch is carried out with an SF₆/HBr/O₂ mixture so as to form deep trenches 8 (FIG. 3). The trenches are provided in such a way that the width, at the level of the surface 4, may be comprised approximately between 0.8 and 2 μm, for example, 1 μm, while the width on the bottom may be approximately between 0.2 and 1.4 μm. The height of the trenches 8 varies from approximately 5 to 50 μm, and the distance between two trenches is approximately between 2 and 6 μm. The angle between the wall of the trenches 8 and the surface 4 of the epitaxial layer 3 may be moreover approximately between 85° and 89.5°.

Next, in order to eliminate the reaction products that adhere to the inside of the trenches 8, a first washing may be performed in a number of steps:

-   -   first step using H₂SO₄+H₂O₂;     -   second step using NH₄OH+H₂O₂; and     -   third step using HCl+H₂O₂.

Then, the wafer 1 may be subject to a first annealing at a high temperature in argon environment and in a dedicated (multi-wafer) reactor in order to eliminate the damage introduced by dry etching. In particular, the first annealing may be performed using the following parameters:

-   -   pressure: approximately 40-760 Torr;     -   argon flow: approximately 30-300 l/min;     -   temperature of the wafer 1: approximately 1000-1150° C.; and     -   treatment time: approximately 30 s-10 min.

In this step, the annealing at high temperature in inert environment may lead to exposure, on the bottom of the trenches 8, of planes <100> and <130> and, along the side walls, of plane <010>, as may be seen from FIG. 4 and, in particular, from the enlarged detail. The trenches 8 may thus assume a new shape and are designated by 8 a in FIG. 4 and the subsequent figures.

Next, the wafer 1 may undergo a second washing in a number of steps:

-   -   first step with H₂SO₄+H₂O₂;     -   second step with NH₄OH +H₂O₂;     -   third step with HCl+H₂O₂; and     -   fourth step with HF.

The wafer 1 may then be subjected to a second annealing in H₂ in a single-wafer LP-CVD chamber so as to eliminate any trace of native oxide. The parameters for this process may be:

-   -   pressure: approximately 40-400 Torr;     -   argon flow: approximately 30-60 l/m;     -   temperature of the wafer: approximately 800-950° C.; and     -   treatment time: approximately 5-10 m.

Next, in the same LPCVD chamber where the second annealing was performed, the trenches 8 may be filled, via selective epitaxial growth, with filling regions having a P doping obtained with boron. The epitaxial growth occurs by supplying flows of dichlorosilane (constituting a gas containing silicon) and diborane, and control of the doping may be obtained by maintaining a constant gradient in the flow of diborane, with a constant flow of dichlorosilane. In addition, the flow of diborane may be decreased with the time of growth. In particular, the intervals of values of the parameters for this step may be:

-   -   pressure approximately within the range 10-400 Torr, such as         30-200 Torr or 60-100 Torr, for example, 80 Torr;     -   flow of the H₂ vector approximately within the range 10-50 l,         such as 15-30 l or 22-27 l, for example, 25 l;     -   temperature approximately within the range 750-1150° C., such as         800-1000° C. or 850-950° C., for example, 900° C.;     -   flow of SiH2CL2 approximately constant within approximately the         range 0.2-0.5 slm (standard litres per minute), such as 0.25-0.4         slm, 0.30-0.35 slm, or 0.30-0.35 slm, for example, 0.32 slm;     -   flow of B₂H₆ according to a decreasing approximately linear         ramp, varying from approximately 150 to 20 sccm, such as from         100 to 30 sccm, for example, from 80 to 40 sccm.

Use of a low temperature, together with selectivity, enables onset of a mechanism of growth from bottom up (hereinafter referred to also as “zip process”), which enables complete filling with substantially no voids to be obtained. In fact, in the conditions indicated, the transportation phenomena of the gas are notably less than in a chemical reaction. The evolution of the selective growth is shown in FIGS. 5-8 and leads to formation of filling regions or columns 9 that may be absolutely coplanar with the surface 4 of the substrate (FIG. 8).

In practice, the selective epitaxial growth may be ensured by the presence of the hard mask 5 and of a halogenide gas (HCl, which develops from the reaction SiH₂Cl_(2(g))→Si_((s))+2HCl_((g))), by the low temperature, and by the low process pressure.

The flow of diborane may be such that the final value is always half the initial one. The initial value may be fixed by the target concentration, which may be calculated so as to ensure the charge balance necessary for the structure of the device that is to be obtained with the process described herein. For example, for a pitch (sum of the width of a trench 8 and of the distance between one trench and another) of 6 μm, and an N epitaxial concentration of 1 W/cm, the initial flow of diborane may be 80 sccm.

Finally the hard mask 5 may be removed (FIG. 9).

In this way, a structure may be obtained, which comprises a semiconductor body 1, having a first conductivity type, with a plurality of trenches 8 a, each housing a column 9 of semiconductor material. The columns 9 house dopant ions having a second conductivity type, distributed in a spatially uniform way. The width of the columns 9 may be between 0.8 and 2 μm, the depth may be between 3 and 50 μm, for example, between 5 and 30 μm, and the aspect ratio may be between 1.5 and 62.5. The distance between two columns 9 may be between 2 and 6 μm, and the pitch (sum of the width of a column 9 and of the distance from an adjacent column 9) may be between 1 and 10 μm, for example, between 5 and 10 μm.

According to a second embodiment, no selective growth is performed. This embodiment comprises a series of initial steps, which may be the same as the ones described with reference to FIGS. 1-3 and comprise forming the hard mask 5 and digging the trenches 8 until the structure of FIG. 3 is obtained.

Next, in order to eliminate the reaction products that adhere to the inside of the trenches 8 and remove the hard mask 5, prior to epitaxial growth, a multistep washing may be performed:

-   -   first step: H₂SO₄+H₂O₂;     -   second step: NH₄OH+H₂O₂;     -   third step: HCl+H₂O₂; and     -   fourth step: HF.

The structure illustrated in FIG. 10 may thus be obtained.

After this step, the wafer 1 may undergo annealing at a high temperature in hydrogen environment in a multi-wafer epitaxial reactor of a barrel type in order to eliminate the damage introduced by the dry etch and any trace of native oxide. The parameters for this process may be:

-   -   pressure: approximately 40-760 Torr;     -   flow of H₂: approximately 30-300 l/m;     -   temperature of the wafer 1: approximately 1000-1150° C.; and     -   treatment time: approximately 1-15 m.

As described for the first embodiment, during the annealing at high temperature in inert environment, the planes <100> and <130> are exposed on the bottom of the trenches 8, and, along the walls, the plane <010>, as shown in the detail of FIG. 11. The trenches 8 a may thus be obtained.

After annealing, in a single-wafer LPCVD epitaxial chamber, the trenches 8 a may be epitaxially filled with silicon with a P doping (boron).

In this case, in view of the absence of the hard mask 5, and of the various process parameters, specified hereinafter, the growth proceeds both on the bottom of the trenches and on the walls, at a higher rate in the proximity of the surface 4 as compared to the bottom. This would produce a premature closing of the trenches 8 a with consequent formation of a wide void within each trench 8 a. For this reason, growth steps may be alternated with etching steps (multi-step process) so as to minimize the number and dimensions of the voids, which may be formed in the area where the two growth fronts come up against one another, starting from the walls, as highlighted by the sequence of FIGS. 12-14. In the example described below, the process comprises four epitaxial growths, separated by three etching steps.

Growth Step

In this step, control of doping is obtained by maintaining a substantially constant gradient in the flow of diborane and by maintaining the flow of dichlorosilane substantially constant. In this case, the flow of diborane increases with the growth time and, for example, the final value of flow of diborane is twice that of the initial flow, which is fixed by the target concentration. The concentration is calculated so as to ensure the charge balance necessary for the structure of the device that is to be made. The ranges of values of the parameters for this step may be:

-   -   pressure approximately within the Torr, for example, 80 Torr;     -   flow of the H₂ vector approximately within the range 10-50 l,         such as 15-30 l or 22-27 l, for example, 25 l;     -   temperature approximately within the range 750-1150° C., such as         800-1000° C. or 850-950° C., for example 900° C.;     -   flow of SiH₂CL₂ approximately constant within approximately the         range 0.2-0.5 slm, such as 0.25-0.4 slm or 0.30-0.35 slm, for         example, slm;     -   flow of B₂H₆ according to an approximately increasing linear         ramp, varying from approximately 20 to 200 sccm, such as from         approximately 50 to 150 sccm, for example, from 60 to 120 sccm;     -   growth time between approximately 150 and 500 s, for example,         300 s.

Etching Step

The range of the parameters for this step may be:

-   -   pressure within approximately the range 10-400 Torr, such as         50-150 Torr or 60-100 Torr, for example, 80 Torr;     -   flow of the H₂ vector approximately within the range 10-50         l,such as 15-30 l or 22-27 l, for example, 25 l;     -   temperature approximately within the range 750-1250° C., such as         1000-1200° C. or 1050-1150° C., for example, 1100° C.;     -   flow of HCl approximately between 0.1 to 2 1, such as 0.2-0.8 l         or 0.4-0.6 l, for example, 0.5 l;     -   etching time approximately between 30 and 150 s, such as between         60 and 120 s, for example, 90 s.

FIGS. 12-14 show various steps of the multi-step process and, namely, FIG. 12 shows the structure after a first growth step; in this step growth of the filling regions 9 starts, with simultaneous formation of surface portions 9 a, which would risk, together with the portions growing on the walls of the trenches 8 a near the surface 4, choking the top part of the trenches 8 a. The growth step is then followed by an etching step, and the structure obtained is shown in FIG. 13, wherein the surface portions 9 a and part of the top portion of the filling regions 9 have been removed. FIG. 14 shows the structure at the end of the growth steps, wherein the filling regions 9 have filled completely the trenches 8 a, and the surface portions 9 a are present.

In practice, in the embodiment of FIGS. 10-14, the process of growth occurs also on the surface, which, at the end of the process, has a characteristic horizontal layer of a P type with grooves set at the openings of the ex-trenches. The horizontal P layer, designated again by 9 a, is present also on the periphery of the device (not shown) where there are no columns. In this case, the P layer 9 a can be used as ring layer of the edge structure of the device that is to be made.

Alternatively, when a planar surface is required, the structure of FIG. 14 may be subjected to a planarization, for example via chemical-mechanical polishing (CMP) so as to expose the surface 4 of the wafer, as shown in FIG. 15.

Some embodiments of power devices based upon the structure with charge-balance trenches obtained according to FIGS. 1-9 or FIGS. 10-15, are described hereinafter, in the latter case, after planarizing the wafer 1.

In particular, for a same thickness of the epitaxial layer, the depth of the trenches determines the class of voltage of the final device: for example, trenches 8 (and hence filling regions 9) having a depth of 5 μm enable devices to be obtained capable of withstanding voltages of up to 100 V, whereas trenches 8 having a depth of 30 μm enable devices capable of withstanding voltages of up to 600 V to be obtained. In general, the solutions described herein may be suitable for devices with breakdown voltages of approximately between 100 and 1700 V.

FIGS. 16-20 represent only one surface portion of the wafer 1, which contains two strips and is highlighted in FIG. 15 by a dashed rectangle 15.

Initially, in a way not shown, an edge region of the device may be formed along the perimeter of the device, in a per se known manner.

Then, in the active area, an oxide layer 16 (gate oxide) having a thickness approximately between 20 nm and 150 nm is first thermally grown, and then a polysilicon layer 17 having a thickness of approximately between 200 and 700 nm is deposited. The polysilicon layer 17 and oxide layer 16 are etched using a resist mask aligned by appropriate zero-layer techniques so as to form openings 18 overlying the filling regions 9, referred to also as columns 9 (FIG. 17). Alternatively, the oxide layer may even not be etched, and the subsequent implantations are made through said oxide.

Using the openings 18, a body implantation is performed; in particular, boron may be implanted with an implantation dose of approximately between 1xe¹³ and 1xe¹⁴ atoms/cm² and energy of approximately between 80 and 200 keV. After a thermal diffusion process has been carried out, the structure of FIG. 18 is obtained, wherein the body regions of a P+ type are designated by 19.

Then (FIG. 19), using a mask (not shown), a deep body implantation is performed; in particular, boron may be implanted with an implantation dose of approximately between 5xe¹⁴ and 5xe¹⁵ atoms/cm² and energy of approximately between 80 and 300 keV. After a thermal activation process has been carried out, the structure of FIG. 19 is obtained, wherein the deep-body regions of a P++ type are designated by 20.

Next (FIG. 20), using a mask (not shown), a source implantation is performed; in particular, phosphorus or arsenic may be implanted with an implantation dose approximately between 5xe¹⁴ and 5xe¹⁵ atoms/cm² and energy approximately between 60 and 200 keV so as to form source regions 21 within the body regions 19. An intermediate dielectric layer 22 may then be deposited with a thickness between 500 nm and 1 μm, and a thermal activation process is carried out. Then the contacts are opened, and the structure of FIG. 20 is obtained. The process of etching the intermediate dielectric layer 25 may be entirely dry or be for a first part a wet etch and for a second part a dry etch.

Next, through a sputtering process, metallization of the front of the wafer 1 may be obtained. Then, a metal layer 26 may be formed, the thickness whereof depends upon the current-carrying capacity required for the device (FIG. 21).

The process proceeds with final known steps, including etching the metal layer 26, depositing a passivation layer, and a subsequent step of masking and etching the passivation layer. Finally, the process of finishing of the back follows.

FIG. 22 shows a variant of the structure of FIG. 21, wherein the columns 9 extend throughout the thickness of the epitaxial layer 2 and terminate at the bottom within the substrate 2.

FIG. 23 shows a variant of the structure of FIG. 21, wherein a buffer layer 30 of N type and resistivity approximately between 100 mΩcm and 5 Ωcm extends between the substrate 2 and the epitaxial layer 3. In this case, the columns 9 terminate within the buffer layer 30.

Advantages of the described process are outlined hereinafter. First, the process may ensure a high doping uniformity in all directions, whether vertical or horizontal, thanks to the variation in the flow of the gases containing silicon. This variation, in a decreasing direction in case of selective growth and in an increasing direction in case of non-selective growth, may be, in fact, fundamental for obtaining charge balance, and hence a proper operation of charge-balance devices, in so far as spatial fluctuations in the charge of the columns 9 entail a reduction in the production yield as well as a reduction in the voltage that can be withstood by the finished devices.

In addition, the processes described may be particularly simple to implement as compared to processes that require a plurality of steps of epitaxial growth, and hence enables a reduction in the cycle time. This means that the costs for manufacturing the devices may be considerably lower than those of known processes.

Furthermore, as compared to processes that require a plurality of steps of epitaxial growth, these processes may give an epitaxial layer of a much better quality, with reduced defectiveness.

In particular, it is emphasized that the first embodiment may be able to prevent, in a repeatable and reliable way, formation of voids. In the case of the second embodiment, empty areas may remain within the structure having dimensions much smaller than those that are obtained with known processes. In particular, tests have shown that with the process described two microbubbles are obtained, one of which in the proximity of the bottom and the other approximately half way up, and a line of suture in a median position. These imperfections are in any case much smaller than those obtainable with known processes and practically have no effect from the standpoint of charge balance, and hence of the electrical behavior of the finished device from the standpoint of charge balance.

With the described processes, it is possible to obtain devices of different classes of voltage (from mean voltages 100-150 V up to 1000-1700 V) by varying only the depth of the trenches, and hence with substantially the same costs.

In addition, it enables improvements in performance in so far as is possible to increase the density of the strips (further reducing the resistivity of the epitaxy) by appropriately varying the process of epitaxial filling.

Finally, it is clear that numerous modifications and variations can be made to the process and device described and illustrated herein, all falling within the scope of the invention. For example, even though the process described refers to a wafer of an N type provided with columns of a P type, it is possible to obtain dual structures. For example, a system, such as an automobile, may include a power device as described above. 

What is claimed is:
 1. A semiconductor power device comprising: a first semiconductor layer of a first conductivity; a second semiconductor layer of the first conductivity adjacent the first semiconductor layer; a trench in said first semiconductor layer and extending into the second semiconductor layer; and a first column including semiconductor material within the trench and having a spatially uniform distribution of dopant ions of a second conductivity type, said first column having at least one void therein.
 2. The semiconductor power device according to claim 1, further comprising at least one second column spaced apart from said first column.
 3. The semiconductor power device according to claim 2, further comprising insulated-gate regions extending above the first semiconductor layer, between said first and at least one second columns.
 4. The semiconductor power device according to claim 3, further comprising body regions of the second conductivity type extending in the first semiconductor layer, said body regions being aligned to and having a doping level greater said columns.
 5. The semiconductor power device according to claim 4, further source regions of the first conductivity type extending within the body regions.
 6. The semiconductor power device according to claim 1, wherein crystalline planes <100> and <130> are exposed on the bottom of the trench, and crystalline plane <010> is exposed on the side walls.
 7. The semiconductor power device according to claim 1, wherein the first semiconductor layer has a first dopant concentration and the second semiconductor layer has a second dopant concentration higher than the first dopant concentration.
 8. A semiconductor power device, comprising: a semiconductor body having a first conductivity type; a trench in said semiconductor body; and a first column of semiconductor material within said trench, said column housing dopant ions having a second conductivity type, wherein said first column has a spatially uniform distribution of said dopant ions and said first column includes at least one void along a center line of the trench.
 9. The device according to claim 8, further comprising: a second column spaced apart from said first column; and an insulated-gate region extending above said semiconductor body, between said first and second columns;
 10. The device according to claim 9, further comprising: body regions of said second conductivity type extending in said semiconductor body, said body region being aligned to and having a doping level greater said first and second columns, respectively; source regions of said first conductivity type extending within said body regions; and metal contact regions extending above said semiconductor body and said insulated-gate regions and in electrical contact with said body regions and said source regions.
 11. The device according to claim 9, wherein the first and second columns are separated from each other by a distance that is between 2 and 6 μm and a sum of a width of the first column and of the distance from between the first and second columns is between 5 and 10 μm.
 12. The device according to claim 8, wherein the first semiconductor layer has a first dopant concentration and the second semiconductor layer has a second dopant concentration higher than the first dopant concentration.
 13. A semiconductor power device comprising: a first semiconductor layer of a first conductivity having a first dopant concentration; a second semiconductor layer of the first conductivity adjacent the first semiconductor layer and having a second dopant concentration higher than the first dopant concentration; a trench in said first semiconductor layer and partially extending into the second semiconductor layer; and a first column comprising semiconductor material within the trench and having a spatially uniform distribution of dopant ions of a second conductivity type; wherein the filler includes at least one void along a center line of the trench.
 14. The semiconductor power device according to claim 13, further comprising a second column spaced apart from said first column.
 15. The semiconductor power device according to claim 14, further comprising an insulated-gate region extending above the first semiconductor layer, between said first and second columns.
 16. The semiconductor power device according to claim 15, further comprising body regions of the second conductivity type extending in the first semiconductor layer, said body regions being aligned to and having a doping level greater said first and second columns, respectively.
 17. The semiconductor power device according to claim 16, further comprising: source regions of the first conductivity type extending within the body regions; and metal contact regions extending above the first semiconductor layer and the insulated-gate regions and in electrical contact with the body regions and the source regions.
 18. The device according to claim 14, wherein the first and second columns are separated from each other by a distance that is between 2 and 6 μm and a sum of a width of the first column and of the distance from between the first and second columns is between 5 and 10 μm.
 19. The semiconductor power device according to claim 13, wherein crystalline planes <100> and <130> are exposed on the bottom of the trench, and crystalline plane <010> is exposed on the side walls.
 20. The semiconductor power device according to claim 13, wherein the first semiconductor layer has a first dopant concentration and the second semiconductor layer has a second dopant concentration higher than the first dopant concentration. 